Power Control

ABSTRACT

An amplification stage comprising: an input scaling block for scaling an input signal based on an input scaling factor to generate a scaled version of the input signal; a power amplifier for receiving the scaled version of the input signal and for generating an amplified version of said signal; an envelope detector for generating a signal representing the envelope of the input signal; an envelope scaling block for scaling the envelope signal based on an envelope scaling factor to generate a scaled version of the envelope signal; a non-linear mapping block for generating a voltage representative of the supply voltage based on the scaled envelope signal; a modulator for generating a power supply voltage for the amplifier based on the voltage generated by the non-linear mapping block; and a power control block for maintaining a linear relationship between the envelope scaling factor and the input scaling factor.

CROSS-REFERENCE TO RELATED APPLICATIONS

Patent application U.S. Ser. No. 13/372,823, filed Feb. 14, 2012, andPatent application GB 1102621.8, filed Feb. 15, 2011, are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power control in a power amplifierarrangement incorporating an envelope tracking power supply. Theinvention is particularly but not exclusively concerned with radiotransmitter power control in mobile communication systems.

2. Description of the Related Art

A typical radio transmitter of a mobile communication system comprisesbaseband signal processing stages, an RF (radio frequency) up-converter,several amplification stages operating at RF frequencies and a followingRF power amplifier stage (which may comprise one or more poweramplifiers).

Such radio transmitters for mobile communications systems must becapable of setting the RF output power across a wide dynamic range. Forexample, a 3GPP (3^(rd) Generation Partnership Project) WCDMA (widebandcode division multiple access) transmitter must provide a power controlrange of >70 dB, typically from around +23 dBm down to around −50 dBm.

An envelope tracking radio transmitter operates by carefully controllingthe supply voltage to the power amplifier stage, such that the supplyvoltage tracks the instantaneous RF envelope of the signal to beamplified with high accuracy. The instantaneous signal envelope is usedto derive the power amplifier supply voltage. The mapping between signalenvelope and supply voltage is designed to achieve certain performanceobjectives. This mapping is implemented using a non-linear transferfunction, often termed a ‘shaping table’

To date envelope tracking power supply systems for power amplifiers havefocused on applications operating with a low power control range.

It is an aim of the invention to provide an improved envelope trackingpower supply which provides for efficient operation with a high powercontrol range.

SUMMARY OF THE INVENTION

There is provided an amplification stage comprising: an input scalingblock for scaling an input signal in dependence on an input scalingfactor to generate a scaled version of the input signal; a poweramplifier for receiving the scaled version of the input signal and forgenerating an amplified version of said signal; an envelope detector forgenerating a signal representing the envelope of the input signal; anenvelope scaling block for scaling the envelope signal in dependence onan envelope scaling factor to generate a scaled version of the envelopesignal; a non-linear mapping block for generating a voltagerepresentative of the supply voltage in dependence on the scaledenvelope signal; a modulator for generating a power supply voltage forthe amplifier in dependence on the voltage generated by the non-linearmapping block; and a power control block for maintaining a linearrelationship between the envelope scaling factor and the input scalingfactor.

The power control block may receive power control information, andgenerates the envelope scaling factor and the input scaling factor independence thereon.

The envelope scaling block may be a multiplier for multiplying theenvelope signal by the envelope scaling factor.

The input scaling block may be a variable gain amplifier, the gain ofthe variable gain amplifier being controlled in dependence on the inputscaling factor.

There may be further provided a mapping block for distributing the gaincontrol information to two or more variable gain stages.

The envelope signal is further scaled in dependence on the differencebetween the voltage level at the output of the non-linear mapping blockand the output of the voltage modulator.

The amplification stage may further comprise: a variable gain amplifierconnected between the output of the non-linear mapping block and theinput of the voltage modulator, the gain of the variable amplifier beingdetermined in dependence on a difference between the voltage level atthe output of the non-linear mapping block and the output of the voltagemodulator.

The gain of the input scaling block may be further determined independence on the difference between the voltage level at the input ofpower amplifier and the voltage level defined by the power controlinformation.

The invention provides a method in an amplification stage, comprising:scaling an input signal in dependence on an input scaling factor togenerate a scaled version of the input signal; generating an amplifiedversion of scaled version of the input signal; generating a signalrepresenting the envelope of the input signal; scaling the envelopesignal in dependence on an envelope scaling factor to generate a scaledversion of the envelope signal; generating a voltage representative ofthe supply voltage in a non-linear mapping block in dependence on thescaled envelope signal; generating a modulated power supply voltage forthe amplifier in dependence on the voltage generated by the non-linearmapping block; and maintaining a linear relationship between theenvelope scaling factor and the input scaling factor.

The method may further comprise generating the envelope scaling factorand the input scaling factor in dependence on power control information.

The method may further comprise multiplying the envelope signal by theenvelope scaling factor.

The method may further comprise controlling the gain of a variable gainamplifier in dependence on the input scaling factor.

The method may further comprise distributing the gain controlinformation to two or more variable gain stages using a mappingfunction.

The may further comprise scaling the envelope signal in dependence onthe difference between the voltage level at the output of the non-linearmapping block and the output of the voltage modulator.

The amplification stage may further comprise scaling the input signalfurther in dependence on the difference between the voltage level at theinput of power amplifier and the voltage level defined by the powercontrol information.

The input path is preferably an RF input path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying Figures, in which:

FIG. 1 illustrates the incorporation of power control in the RF path ofan envelope tracking power supply in accordance with prior arttechniques;

FIG. 2 illustrates the incorporation of power control in the envelopepath of an envelope tracking power supply in accordance with anembodiment of the invention; and

FIG. 3 illustrates the incorporation of power control in the envelopepath of an envelope tracking power supply incorporating closed-looppower control in accordance with another embodiment of the invention.

DESCRIPTION OF THE PREFEERED EMBODIMENTS

The present invention is described herein by way of reference toparticular advantageous arrangements, embodiments and implementations.The invention is not limited to the details of such arrangements. Inparticular the invention is described in relation to an envelopetracking power supply for an RF power amplifier of a mobilecommunications system in which power control is provided. Thisrepresents an exemplary implementation, and the invention is not limitedto such an implementation.

FIG. 1 illustrates an envelope tracking system incorporating powercontrol for the signals to be amplified.

A baseband processing subsystem 102 provides baseband data to betransmitted to one input of a multiplier 104. Power control data, suchas an uplink power control level data, is provided from a power controlblock 106 to a second input of the multiplier 104. The multiplier 104generates at its output the baseband data to be transmitted adjustedaccording to the power control data.

The power-adjusted baseband data at the output of the multiplier isprovided as an input to a baseband to RF up-converter stage 112. The RFsignal at the output of the RF up-converter stage 112 is provided as aninput to an amplification stage 114 comprising a plurality ofamplifiers, in the illustrated example three amplifiers 114 a, 114 b,114 c. The amplification stage 114 provides pre-amplification of the RFsignal to provide the input to an RF power amplifier 118 on line 122.The RF power amplifier 118 provides an RF output signal on its output online 124.

In accordance with a preferred implementation of an envelope trackingpower supply, the output of the multiplier 104 is also provided as aninput to an envelope detector 110. The envelope detector generates anenvelope signal, representing the envelope of the baseband signal to beamplified, at its output.

The envelope signal is provided as an input to a shaping table 108. Theshaping table 108 implements a non-linear mapping function between itsinput and output to provide the shaped input to an envelope trackingmodulated power supply 116. The envelope tracking modulated power supply116 provides a power supply voltage on line 120 to a power supplyterminal of the power amplifier 118, which voltage tracks the envelopeof the signal to be amplified in order to provide efficient operation ofthe power amplifier 118.

The shaping table 108 determines an instantaneous output voltage for theenvelope tracking modulated power supply 116 in dependence oncharacteristics of the instantaneous envelope signal. The shaping table108 has an x-axis representing RF input voltage, and a y-axisrepresenting voltage supply. Thus for a given RF envelope voltage acorresponding power amplifier voltage supply level is determined.

With reference to FIG. 2, there is illustrated an improvement to theprior art arrangement of FIG. 1 to utilise power control information inaccordance with an embodiment of the invention. Reference numerals inFIG. 2 which correspond to reference numerals in FIG. 1 denote elementsin FIG. 2 corresponding to elements in FIG. 1.

The baseband processing subsystem 102 generates I data and Q datasignals in respective sub-blocks 202 and 204. In the path to the inputof the RF amplifier 118 (the RF path), the I data signal is connected toa delay block 206 and the Q data signal is connected to a delay block208. In the path to the envelope tracking modulated power supply 116(the envelope path), the I data signal is connected to a delay block 228and the Q data signal is connected to a delay block 230.

The delay blocks 228, 230, 206 and 208 are optionally provided to enabledelay balance between the RF path and the envelope path. The control ofthe delay blocks is not described herein, as it falls outside the scopeof the present invention and will be familiar to one skilled in the art.Typically the delay blocks are provided (or enabled) in only one orother of the RF path or the envelope path.

In the RF path to the RF amplifier, after the optional delay block 206the I data signal is provided as an input to an analogue-to-digitalconverter 210, and then the generated analogue signal is reconstructedin a reconstruction filter 214 for the analogue signal based on the Idata signal. After the delay block 208 the Q data signal is provided asan input to an analogue-to-digital converter 212, and then the generatedanalogue signal is reconstructed in a reconstruction filter 216 for theanalogue signal based on the Q data signal.

An RF power control path comprises the RF up-converter 112 and theamplification stage 114. The RF up-converter 112 includes a variablegain amplifier 218 connected to receive the analogue signal based on theI data signal from the reconstruction filter 214, and a variable gainamplifier 220 connected to receive the analogue signal based on the Qdata signal from the reconstruction filter 216. As discussed furtherhereinbelow, each of the variable gain amplifiers 218 and 220 furtherreceive variable gain control signals.

The respective outputs of the variable gain amplifiers 218 and 220 driverespective quadrature mixers 222 and 224. The quadrature mixer 222,associated with the I data signal, receives an oscillator signal from alocal oscillator 119 (for up-conversion). The quadrature mixer 224,associated with the Q data signal, receives the oscillator signal fromthe local oscillator 119, shifted 90° by phase shifter 121.

The outputs of the quadrature mixers 222 and 224 are combined to formthe input for the amplification stage 114.

The amplification stage 114 is denoted as a variable gain amplifier,which may comprise a plurality of amplifier stages as in FIG. 1, andamplifies the combined output of RF up converter 112 to provide theinput signal for amplification by the RF power amplifier 118.

An RF power control block 107 a receives power control information onsignal line 250. The RF power control block 107 a is adapted to providepower control information for the RF input path. The RF power controlblock receives the power control information, and generates a version ofthe power control information for use in the RF input path. The RF inputpower control block generates the power control information on signallines 250 in a form for delivery to the RF input path, which in theexample shown is a signal to be applied to the mapping block 226.

More specifically, in the arrangement of FIG. 2 the RF power controlblock 107 a provides power control information to a mapping block 226.The mapping block 226 generates first power control information data tocontrol the variable gain of the baseband variable gain amplifiers 218and 220, and second power control information data to control thevariable gain of the amplification stage 114.

The mapping block 226 includes a function, such as a table, forgenerating control signals to the variable gain amplifiers for a givenset of power control information. In a mobile communications system,where the arrangement of FIG. 2 is implemented in a handset, the levelset by the RF power control block 107 a is controlled by informationtransmitted to the handset from the network infrastructure.

In the path to the envelope tracking modulated power supply 116, afterthe optional delay stages 228 and 230 the I and Q data signals areprovided as inputs to the envelope detector 110. In the illustratedembodiment the envelope detector includes a CORDIC (COordinate RotationDIgital Computer) function for generating the envelope signal. CORDIC isa simple and efficient algorithm to calculate hyperbolic andtrigonometric functions. It is commonly used when no hardware multiplieris available as the only operations it requires are addition,subtraction, bit-shift and table lookup. More generally the envelopedetector may include an “abs” function, CORDIC being a particularimplementation of this.

An envelope power control block 107 b receives the power controlinformation on signal line 250 (which is also provided to the powercontrol block 107 a). The envelope power control block 107 b is adaptedto provide power control information for the envelope path. The envelopepower control block receives the power control information, andgenerates a version of the power control information for use in theenvelope path. The envelope power control block generates the powercontrol information on signal lines 250 in a form for delivery to theenvelope path, which in the example shown is a signal to be applied tothe multiplier 234.

The RF and envelope power control blocks 107 a and 107 b thus receivecommon power control information, and independently generate a linearlyscaled version of it, which is delivered to be multiplied with therespective RF and envelope signals (via a mapping means in the RF path)in order to scale the RF and envelope signals to follow the powercontrol information.

The envelope signal at the output of the envelope detector 110 isprovided as a first input to a multiplier 234. The second input of themultiplier 234 is the power control information provided by the envelopepower control block 107 b. The power control information provided to theenvelope power control block 107 b is linearly related to the powercontrol information provided to the RF power control block 107 a.

The scaled envelope signal from the multiplier 234 on line 236 providesthe input to the shaping table 108. The output of the shaping table isprovided as an input to a digital-to-analogue converter 238, and thethus generated analogue signal is processed in a reconstruction filter240 and then provided as the input to the envelope tracking modulatedpower supply 116. The envelope tracking modulated power supply thenprovides the power supply voltage to the RF amplifier 118 on line 120 independence on the output voltage value provided by the shaping table.

The power control provided in the RF path to the RF amplifier isimplemented in the analogue domain in the arrangement of FIG. 2, inaccordance with prior art techniques.

In the envelope path, power control is implemented in the digitaldomain. This is determined by a requirement to implement the scaling ofthe envelope signal before the shaping table. If the scaling of theenvelope signal were implemented after the shaping table, i.e. byscaling the power amplifier supply voltage itself (the voltage from theshaping table), this would not achieve the same non-linear mapping owingto the characteristics of the shaping table. The scaling in the envelopepath may therefore be implemented at any point between the basebandconversion of the signals and the input to the shaping table.

Preferably the baseband signals are maintained as large as possible forsignal processing, and therefore the shaping of the envelope signal ispreferably implemented as close to the input of the shaping table aspossible.

The invention thus operates by scaling the envelope voltage fed into theshaping table. In the illustrated embodiment this scaling is thereforedone by scaling the envelope signal itself before it passes to theshaping table. In an alternative arrangement this may be done by scalingthe baseband I-Q voltages prior to calculation of the envelope. Gaincontrol in the main RF path is still preferably implemented, as shown inFIG. 2 with amplifiers 114.

The scaling of the envelope signal in dependence on power controlinformation determines the range of the shaping table which can beutilised. Power control information which determines a reduced power,for example, limits the upper range of the shaping table which isaccessed.

Thus the envelope path is adapted to scale the voltage in the envelopepath depending on the target average output power based on a powercontrol signal delivered on signal line 250.

Specifically, the envelope voltage fed into the ‘shaping table’ 108 isscaled with output power information.

For example, in a mobile communication system implementation, where thearrangement of FIG. 2 is implemented in a handset, if the networkdemands a 20 dB power reduction, and transmits an appropriate powercontrol command to the handset, the envelope voltage fed into themultiplier 234 is multiplied by a factor of 0.1 before being deliveredto the shaping table 108. Thus the envelope power control block 107 bgenerates the appropriate scaling signal for the multiplier 234 independence upon the power control information on signal line 250. At thesame time, based on the same power control information delivered onsignal line 250 to the RF input power control block 107 a, the RF inputvoltage must be also be multiplied by a factor of 0.1. This is achievedby controlling the composite gain of the baseband VGAs 218 and 220 andthe RF VGA 114.

Thus it can be understood in general that the signals in the envelopeand RF paths are both scaled by the same amount based on received powercontrol information, with the scaling factor applied in any particularstage being linearly related to an overall scaling objective determinedby common power control information.

The present invention extends an envelope tracking power supply systemto provide improved operation across a range of average output powers.The power control information provided to the envelope path (and the RFpath) preferably represents the current average output powerrequirements.

In a terminal of a mobile communication system, the terminal can adjustits transmit power in response to commands issued by the network, bychanging the gain in the RF and envelope paths as discussed.

The embodiments of the invention described herein illustrate a solutionof implementing average tracking RF power control in radio transmittersusing feed-forward envelope tracking, but is also applicable inclosed-loop arrangements.

FIG. 3 illustrates an embodiment of the invention in a closed loopimplementation.

In FIG. 3, the arrangement of FIG. 2 is modified to include feedback toallow for power control in a closed loop implementation. If feedbackwere only taken from the output of the RF power amplifier 118 in orderto implement the closed loop implementation, it is not possible todirectly determine whether the error in the level of the output signalresults from gain errors in the envelope path or gain errors in the RFpath. This problem may be overcome by taking feedback from two points,so as to be able to distinguish the path in which a gain error arises,and provide feedback to control the gain of that path. FIG. 3 representsa particular way of implementing this, in which feedback for theenvelope path is taken from the supply fed to the power amplifier at theoutput of the envelope tracking modulator 116, and feedback from the RFpath is taken from the input of the RF power amplifier on line 122.

In order to provide closed loop control in the RF path gain, a feedbackarrangement comprising RF envelope detector 302, error amplifier 304,combiner 303 and combiner 305 is provided. A power coupler 306 isconnected to detect the power on the input to the power amplifier online 122. The power coupler 306 delivers the detected power signal tothe RF envelope detector 302. The RF envelope detector 302 generates anenvelope signal representing the envelope of the signal at the input tothe RF power amplifier, and delivers this as one input to an erroramplifier 304. The error amplifier 304 further receives an input fromthe power control block 107 a, representing the power controlinformation currently being delivered by the power control block 107 a.The error amplifier 304 amplifies an error corresponding to thedifference in the signals received at its two inputs, and generates tworespective outputs to the combiner blocks 303 and 305. The output of theerror amplifier 304 provided to the combiner 303 is combined with theoutput of the mapping block 226 for the variable gain amplifier 114, inorder to generate a modified variable gain control signal for the RFvariable gain amplifier 114. The output of the error amplifier 304provided to the combiner 305 is combined with the output of the mappingblock 226 to generate a modified variable gain signal for the basebandvariable gain amplifiers 218 and 220.

In this way, any error in the gain of the RF path can be removed. Itshould be noted that this includes errors due to non-linearities in thegain control characteristics of the baseband amplifiers 218 and 220 andthe RF variable gain amplifier 114.

In order to provide for closed loop operation in the envelope path, afeedback arrangement is provided including an analogue-to-digitalconverter 241, an error amplifier 239, and a variable gain amplifier237. The variable gain amplifier 237 is connected between the output ofthe shaping table 108 and the input to the digital-to-analogue converter238. The analogue-to-digital converter 241 is connected to receive thesignal at the output of the envelope tracking modulator 116, or thesignal at the power supply input to the amplifier 118, on line 120. Thisconverted digital signal is delivered as one input to the erroramplifier 239. The other input to the error amplifier is provided by theoutput of the shaping table 108. The error amplifier amplifies thedifference between the signals at its two inputs in order to control thegain of the variable amplifier 237.

In this way any error in the gain of the envelope path through to theoutput of the envelope tracking modulator 116 is removed.

It will be understood that variations to the specific points at whichthe signals of the RF power amplifier are sensed in order to providefeedback may be varied. As noted above, it is undesirable to only detectthe output of the RF power amplifier because this does not provideinformation as to which path the error is in. However if the error inone path is detected and corrected, then the other path may be correctedbased on making a detection of the output of the RF power amplifier. Forexample, in the arrangement of FIG. 3, the closed loop arrangement inthe envelope path removes any error in the signal provided on the supplyvoltage line 120 to the RF power amplifier. In this case it may beassumed that any error in the output of the RF power amplifier on line124 is associated with the RF input path, and therefore the powercoupler 306 may be connected on output line 124 rather than output line122.

The implementation of the RF envelope detector 302 falls outside thescope of the present invention, and one familiar with the art willappreciate how such implementation may be achieved. It is envisaged thatthe RF envelope detector 302 may include a receive chain, ananalogue-to-digital converter, and a magnitude detector, to provide adigital signal to the input of the error amplifier representing theenvelope of the input signal to the RF amplifier.

In the arrangement of FIG. 3, the power control information signal ispreferably adjusted based on the feedback signal, so that one scaling isstill applied to the envelope signal, rather than two scalings beingapplied to the envelope signal.

The invention has been described herein by way of example with referenceto exemplary arrangements of an envelope tracking modulated power supplyfor a power amplifier, and with particular reference to implementationin a handset of a mobile communication system. One skilled in the artwill appreciate that the invention is not limited to such animplementation. For example, the invention could be implemented in thenetwork side of a mobile communication system, such as in a basestation. Network planning load-balance considerations to suit localtraffic conditions could determine the power control information, whichcould be assessed dynamically. The invention is further not limited tomobile communication applications.

In the Figures the power control blocks 107 a and 107 b are shown asreceiving common power control information on signal lines 250. Thispower control information may be provided in accordance with variousarrangements, some of which are described hereinabove. Each powercontrol block 107 a and 107 b adapts the received power controlinformation in order to provide it in an appropriate form of data forapplication in the respective envelope or RF path. Whilst the powercontrol blocks 107 a and 107 b adapt the power control information tothe appropriate form the for the respective paths, the power controlblocks 107 a and 107 b are linearly related to each other such that alinear relationship is maintained between the scaling factors applied inthe respective paths.

The present invention has been described herein by way of example, withreference to particular non-limiting embodiments. Whilst the inventionhas particular applicability in mobile communication systems, and inparticular mobile communication handsets, the invention is not limitedto such applications. One skilled in the art will appreciate the moregeneral applicability of the invention and its embodiments as describedherein, and the scope of protection afforded by the invention as definedby the appended claims.

What is claimed is:
 1. An amplification stage comprising: an inputscaling block for scaling an input signal in dependence on an inputscaling factor to generate a scaled version of the input signal; a poweramplifier for receiving the scaled version of the input signal and forgenerating an amplified version of said signal; an envelope detector forgenerating a signal representing the envelope of the input signal; anenvelope scaling block for scaling the envelope signal in dependence onan envelope scaling factor to generate a scaled version of the envelopesignal; a non-linear mapping block for generating a voltagerepresentative of the supply voltage in dependence on the scaledenvelope signal; a modulator for generating a power supply voltage forthe amplifier in dependence on the voltage generated by the non-linearmapping block; and a power control block for maintaining a linearrelationship between the envelope scaling factor and the input scalingfactor.
 2. The amplification stage of claim 1 wherein the power controlblock receives power control information, and generates the envelopescaling factor and the input scaling factor in dependence thereon. 3.The amplification stage of claim 1 wherein the envelope scaling block isa multiplier for multiplying the envelope signal by the envelope scalingfactor.
 4. The amplification stage of claim 1 wherein the input scalingblock is a variable gain amplifier, the gain of the variable gainamplifier being controlled in dependence on the input scaling factor. 5.The amplification stage of claim 4 wherein there is further provided amapping block for distributing the gain control information to two ormore variable gain stages.
 6. The amplification stage of claim 1,wherein the envelope signal is further scaled in dependence on thedifference between the voltage level at the output of the non-linearmapping block and the output of the voltage modulator.
 7. Theamplification stage of claim 6, further comprising: a variable gainamplifier connected between the output of the non-linear mapping blockand the input of the voltage modulator, the gain of the variableamplifier being determined in dependence on a difference between thevoltage level at the output of the non-linear mapping block and theoutput of the voltage modulator.
 8. The amplification stage of claim 1,wherein the gain of the input scaling block is further determined independence on the difference between the voltage level at the input ofpower amplifier and the voltage level defined by the power controlinformation.
 9. A method in an amplification stage, comprising scalingan input signal in dependence on an input scaling factor to generate ascaled version of the input signal; generating an amplified version ofscaled version of the input signal; generating a signal representing theenvelope of the input signal; scaling the envelope signal in dependenceon an envelope scaling factor to generate a scaled version of theenvelope signal; generating a voltage representative of the supplyvoltage in a non-linear mapping block in dependence on the scaledenvelope signal; generating a modulated power supply voltage for theamplifier in dependence on the voltage generated by the non-linearmapping block; and maintaining a linear relationship between theenvelope scaling factor and the input scaling factor.
 10. The method ofclaim 9 further comprising generating the envelope scaling factor andthe input scaling factor in dependence on power control information. 11.The method of claim 9 wherein further comprising multiplying theenvelope signal by the envelope scaling factor.
 12. The method of claim9 further comprising controlling the gain of a variable gain amplifierin dependence on the input scaling factor.
 13. The method of claim 12further comprising distributing the gain control information to two ormore variable gain stages using a mapping function.
 14. The method ofclaim 9, further comprising scaling the envelope signal in dependence onthe difference between the voltage level at the output of the non-linearmapping block and the output of the voltage modulator.
 15. Theamplification stage of claim 9, further comprising scaling the inputsignal further in dependence on the difference between the voltage levelat the input of power amplifier and the voltage level defined by thepower control information.